The present invention relates to a motor, and more particularly, to a motor for a pressure generating apparatus such as a turbo-molecular pump.
A turbo-molecular pump produces an ultra-high vacuum state and is employed in, for example, semiconductor fabrication related apparatuses (e.g., sputtering apparatuses, chemical vapor deposition (CVD) apparatuses, and etching apparatuses) and measuring apparatuses (e.g., electron microscopes, surface analysis apparatuses, and environment testing apparatuses). A typical turbo-molecular pump includes a rotary shaft and a plurality of rotor vanes fixed to the rotary shaft. The turbo-molecular pump rotates the rotor vanes to produce a molecular flow and to discharge gases. This causes an ultra-high vacuum state in the interior of the apparatus connected to the turbo-molecular pump.
The rotary shaft is rotated at a high speed to produce the ultra-high vacuum state. The bearing that supports the rotary shaft must thus be capable of high speed rotation. A ball bearing, which requires lubricating oil, is not appropriate for such application. This is because the vapor pressure of the lubricating oil, although low, hinders depressurization to the ultra-vacuum state by the turbo-molecular pump. Further, vaporized lubricating oil contaminates vacuum chambers. Accordingly, Japanese Unexamined Utility Model Publication No. 63-14894 and Japanese Unexamined Patent Publication No. 2-16389 describe a turbo-molecular pump that does not use lubricating oil.
FIG. 9 shows a turbo-molecular pump 60, which is described in Japanese Unexamined Patent Publication No. 2-16389. The turbo-molecular pump 60 includes a motor 63 and a housing 67. The motor 63 has a rotary shaft 64 to which a wheel 62 is secured. Rotor vanes 62a extend radially from the wheel 62. A magnetic bearing 61 and an air bearing 66 rotatably support the rotary shaft 64. The magnetic bearing 61 and the air bearing 66 restrict axial and radial movement of the rotary shaft 64. The magnetic bearing 61 is accommodated in the housing 67, which includes a cylinder 67a, and has a plurality of magnets 65 arranged in the cylinder 67a. The magnets 65 are opposed to magnets (not shown) that are embedded in the walls of a bore 62b formed in the wheel 62. The repelling force between the magnets 65 and the magnets of the wheel 62 rotates the wheel 62 about the cylinder 67a without contacting the cylinder 67a. The rotary shaft 64 extends through a case 68. The air bearing 66 is located at the basal end, or lower end, of the rotary shaft 64, which extends from the case 68, and has a dynamic pressure bearing portion 69. The dynamic pressure bearing portion 69 has a plurality of dynamic pressure grooves in the surface opposing the case 68. High speed rotation of the rotary shaft 64 causes the dynamic pressure grooves to form a compressed gas layer, which radially supports the rotary shaft 64.
During operation of the turbo-molecular pump 60, the pressure applied to the upper end of the rotary shaft 64 (the wheel 62) is less than the pressure applied to the lower end of the rotary shaft 64. The pressure difference displaces the rotary shaft 64 axially toward the wheel 62. The displacement results in the rotary shaft 64 (the wheel 62) interfering with the surrounding components and hinders smooth operation of the motor 63. Thus, a no-contact bearing that supports the rotary shaft 64 without interference even when a pressure difference occurs is needed.
Accordingly, the number of air bearings may be increased or a larger air bearing may be employed. Further, in the magnetic bearings, the number of magnets may be increased or a larger magnet may be employed. However, this would make the motor 63 larger and more complicated.
Additionally, the air bearing 66 is arranged outside the motor 63 in the pump 60 of FIG. 9. Thus, the motor 63 and the bearing 66 must be installed on the pump 60. Further, the bearing 66 must be assembled together with the motor 63. This increases the number of steps for manufacturing the pump 60 and complicates manufacturing. In addition, the location of the bearing 66 imposes design restrictions to the pump 60.
Accordingly, a motor that employs only a magnetic bearing has been proposed for turbo-molecular pumps. However, the magnetic bearing must be made of a magnetic material having strong magnetism to be small enough to fit in the motor. Such magnetic material is expensive and increases the cost of the motor.
It is an object of the present invention to provide a compact motor and turbo-molecular pump.
To achieve the above object, the present invention provides a motor including a rotary shaft. An axial urging force is applied to the rotary shaft in a first direction when the motor is driven. A first magnetic bearing supports the rotary shaft. A second magnetic bearing supports the rotary shaft. The first and second magnetic bearings each include two magnets that repel each other. A resultant force of the repulsion of the first magnetic bearing and the repulsion of the second magnetic bearing acts in a direction opposite to the first direction.
A further aspect of the present invention provides a vacuum pump provided with a motor. The motor includes a rotary shaft. An axial urging force is applied to the rotary shaft in a first direction when the motor is driven. A first magnetic bearing supports the rotary shaft. A second magnetic bearing supports the rotary shaft. The first and second magnetic bearings each include two magnets that repel each other. A resultant force of the repulsion of the first magnetic bearing and the repulsion of the second magnetic bearing acts in a direction opposite to the first direction.
Another aspect of the present invention provides a motor including a case, a rotary shaft projecting from the case, and a first magnetic bearing and a second magnetic bearing for supporting the rotary shaft and restricting axial movement of the rotary shaft. The first magnetic bearing includes a first rotated magnet fixed to the rotary shaft to rotate integrally with the rotary shaft and a first fixed magnet fixed to the case opposing the first rotated magnet and separated from the first rotated magnet by a first distance. The first rotated magnet and the first fixed magnet have the same polarity. The second magnetic bearing includes a second rotated magnet fixed to the rotary shaft to rotate integrally with the rotary shaft, and a second fixed magnet fixed to the case opposing the second rotated magnet and separated from the second rotated magnet by a second distance. The second rotated magnet and the second fixed magnet have the same polarity. An area of opposition between the first rotated magnet and the first fixed magnet differs from an area of opposition between the second rotated magnet and the second fixed magnet.
In a further aspect of the present invention, a pressure generating apparatus generates a predetermined pressure. The apparatus includes a motor. The motor includes a rotary shaft having a distal end and a basal end, and a non-contact bearing for supporting the rotary shaft. A vane is rotated integrally with the rotary shaft. A first chamber is located at the distal end of the rotary shaft. A second chamber is located at the basal end of the rotary shaft. A passage connects the first and second chambers.
Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.